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38 AGRONOMY JOURNAL, VOL. 94, JANUARY–FEBRUARY 2002

U.S. Department of Agriculture Agricultural Research Service and vice. 1999. Soil quality card design manual: A guide to developNatural Resources Conservation Service. 1998. Soil quality kit guide locally adapted conservation tools [Online]. Available with updates[Online]. Available with updates at http://www.statlab.iastate.edu/ at http://www.statlab.iastate.edu/survey/SQI/cardguide.html (veri-survey/SQI/kit2.html (verified 26 Sept. 2001). fied 27 Sept. 2001).

U.S. Department of Agriculture Natural Resources Conservation Ser-

Soil Quality for Sustainable Land Management: Organic Matter and AggregationInteractions that Maintain Soil Functions

Martin R. Carter*

ABSTRACT is seen as a basic premise of soil quality (Larson andPierce, 1991, 1994). If a soil is not suitable for a specificSoil quality concepts are commonly used to evaluate sustainableuse, then it is not appropriate to attempt to assign orland management in agroecosystems. The objectives of this review

were to trace the importance of soil organic matter (SOM) in Canadian describe quality for that specific use or function. In manysustainable land management studies and illustrate the role of SOM cases, however, it is not possible to make a perfect matchand aggregation in sustaining soil functions. Canadian studies on soil between the soil and its intended use. Under these cir-quality were initiated in the early 1980s and showed that loss of SOM cumstances, quality must be built into the system usingand soil aggregate stability were standard features of nonsustainable best management scenarios.land use. Subsequent studies have evaluated SOM quality using the Ecosystem concepts such as function, processes, attri-following logical sequence: soil purpose and function, processes, prop-

butes, and indicators, have proved to be a useful frame-erties and indicators, and methodology. Limiting steps in this soilwork to describe soil quality (Larson and Pierce, 1991,quality framework are the questions of critical limits and standardiza-1994; Doran and Parkin, 1994; Doran et al., 1996; Cartertion for soil properties. At present, critical limits for SOM are selectedet al., 1997; Karlen et al., 1997). However, a preciseusing a commonly accepted reference value or based on empirically

derived relations between SOM and a specific soil process or function definition of soil quality proves to be elusive. This is(e.g., soil fertility, productivity, or erodibility). Organic matter frac- probably related to the innate difficulty in defining soiltions (e.g., macro-organic matter, light fraction, microbial biomass, itself and to the multifaceted nature (i.e., scientific, per-and mineralizable C) describe the quality of SOM. These fractions sonal, and social) of environmental concerns. Carter andhave biological significance for several soil functions and processes MacEwan (1996) suggested that although soil qualityand are sensitive indicators of changes in total SOM. Total SOM describes an objective state or condition of the soil, itinfluences soil compactibility, friability, and soil water-holding capac-

also is subjective, i.e., evaluated partly on the basis ofity while aggregated SOM has major implications for the functioningpersonal and social determinations. The above frame-of soil in regulating air and water infiltration, conserving nutrients,work of soil quality has utility when it is directed orand influencing soil permeability and erodibility. Overall, organicfocused towards the manipulation, engineering, and/ormatter inputs, the dynamics of the sand-sized macro-organic matter,

and the soil aggregation process are important factors in maintaining management of the soil resource. Thus, soil quality is aand regulating organic matter functioning in soil. technology, an applied science, directed towards better

soil management.The objective of this paper is to review the context

Soil quality is not a new topic. Early scientific en- and approach to soil quality, with specific emphasis ondeavors recognized the importance of categorizing soil organic matter (SOM) and soil aggregation. Specificsoil type and soil variables or properties in regard to land objectives are to (i) trace the origins of soil qualityor soil use, especially for agricultural purposes (Carter et research in the concern for sustainable land manage-al., 1997). ment in Canada, (ii) differentiate between descriptiveThe impetus to define and assess soil quality is in and functional approaches used to characterize soilmany ways derived from outside of the scientific com- quality, (iii) evaluate the role and limitations of utilizingmunity, being related to the concern of society with the SOM as a key attribute of soil quality, and (iv) assessoverall quality or health of the environment. However, the factors that regulate SOM functioning in soil.due to concerns with soil degradation and the need forsustainable soil management in agroecosystems, there

SUSTAINABLE LAND MANAGEMENThas been renewed scientific attention to characterizeAND SOIL QUALITYsoil quality. Placing a value on soil in regard to a specific

function, purpose, or use leads to the concept of soil Soil quality is considered a key element of sustainablequality. agriculture (Warkentin, 1995). The latter refers to pro-

The basic idea of fitness for use in regard to agricul- ductivity, economic, social, and environmental compo-nents of land use systems (Smyth and Dumanski, 1995).tural use of soil, which was reflected in early and ongoingAlthough sustainability issues are much broader thanattempts at classifying soil suitability or land capability,soil quality, the strong emphasis on maintaining thenatural-resource base ensures that maintaining good

Agric. and Agri-Food Canada, 440 Univ. Ave., Charlottetown, PE, soil quality is an integral part of sustainable agricultureC1A 4N6 Canada. Received 22 May 2000. *Corresponding author(Miller and Wali, 1995).([email protected]).

Abbreviations: SOM, soil organic matter.Published in Agron. J. 94:38–47 (2002).

CARTER: SOIL QUALITY FOR SUSTAINABLE LAND MANAGEMENT 39

Although all of the above components must be satis- much soil quality evaluation. Soil quality indicators infied to meet the goal of sustainable land use, it is recog- landscapes influenced by soil erosion have been studiednized that sustainability has a time scale and that the where the relationship between land use and soil qualitytime scale may differ for each component. Even though is difficult to characterize because of the complexity ofloss of sustainability of any one component would clas- soils and spatial variability. In Saskatchewan, Andersonsify the land use system as unsustainable, in reality, and Gregorich (1984) assessed the effects of erosionshort-term economic viability can be sustained by inputs and cultivation on soil quality attributes while Verityto the system even when use of the natural-resource and Anderson (1990) characterized the influence of ero-base has become nonsustainable. This latter scenario sion on changes in soil quality attributes, including thehas serious implications for SOM and the sustainability relationship between soil quality and crop production.of intensive farming systems, as most economic models Pennock et al. (1994) assessed soil redistribution in land-utilized to describe sustainability in agriculture often dis- scapes of southern Saskatchewan with different cultiva-regard or marginalize the natural-resource component. tion histories and showed that SOM was the major soil

quality indicator influenced by erosion. Other studiesUse of the Concept of Soil Quality in Canadain Saskatchewan have characterized soil quality in dif-

Early work in Canada to assess the quality of agricul- ferent farming systems and noted the benefits of contin-tural soils was conducted within the context of sustain- uous cropping and frequent additions of crop residuesable land use. A series of papers in the Canadian Journal on soil quality in general and SOM quality in particularof Soil Science summarized the effects of intensive culti- (Boehm and Anderson, 1997). In New Brunswick, Caovation on soil quality in different regions of the country. et al. (1994) assessed the cumulative effect of soil ero-Ketcheson (1980) noted that intensive agriculture, espe- sion and redistribution under intensive potato produc-cially monoculture and row crops, had resulted in a tion and found that both soil loss and gain (i.e., deposi-deterioration of SOM levels and soil physical properties tion) were related to SOM. Gregorich et al. (1998)in southern Ontario. An increase in the use of grass– recently reviewed the relation between soil erosion andlegume forages was seen as a solution to this problem. deposition processes and distribution and loss of SOMIn Quebec, Martel and MacKenzie (1980) compared the using the Century model.effect of different land use practices on soil quality andshowed that the conversion of forest soils into agricul- Soil Quality and Crop Productiontural soils was accompanied by a loss in both SOM

Studies have also been conducted in other regionsand structural stability and, under some conditions, anof Canada to elucidate soil quality in the context ofincrease in soil compaction. Use of continuous grasssustainable land use and management. Olson et al.(�5 yr), however, tended to reverse the decline in soil(1996) used a field assay based on removing soil andquality. Saini and Grant (1980) reviewed the deleterioussubsequent deposition of soils with diverse propertieseffect of continuous potato (Solanum tuberosum L.)to identify key soil quality attributes that could alsoculture in New Brunswick and the neighboring state ofbe related to crop productivity in southern Alberta.Maine (USA) and identified decreases in soil structuralSignificant relationships between crop yield and SOM,stability, porosity, and SOM as characteristic featurescarbonate concentration, and pH were obtained. Studiesof declining soil quality. All of these studies recognized,in more northern regions of Alberta have assessed themainly through qualitative assessment of soil properties,influence of integrated cropping systems on soil quality.the major role that SOM plays in the maintenance ofCombinations of herbicide application, crop rotation,soil quality.and tillage systems were found to influence several soilInterest in soil quality was further developed by aquality attributes, especially SOM quality (Soon andseries of technical bulletins published by AgricultureDarwent, 1998). Wani et al. (1994) found that adoptingCanada to describe key parameters that can be used togreen manures and organic amendments in crop rota-measure soil quality in different regions of the country.tions provided a measurable increase in SOM qualityActon (1991) identified soil pH, salinity, sodicity, SOM,and other soil quality attributes compared with continu-and other attributes for use in western Canada whileous cereal systems.Coote (1991) listed soil erosion, acidification, and com-

paction processes in eastern Canada. The effects ofSoil Quality and Forest SoilsSOM on soil quality in relation to soil structure, water

retention, nutrient availability, and other functions were Several studies have assessed soil quality indicatorscovered by Schnitzer (1991). The especial importance in forest soils. Pennock and van Kessel (1997) evaluatedof SOM for soil quality in intensively row-cropped sandy the effect of clear-cut harvest practices on forest soilssoils in eastern Canada was assessed by Mathur (1991). in Saskatchewan and established that although thereMore recently, Angers and Carter (1996) described the was a decline in soil quality attributes, the values werepositive relationship between SOM and water-stable within the natural or undisturbed range, except foraggregates in eastern Canada. SOM, which had declined below the natural range.

McBride et al. (1990), using apparent electrical conduc-Soil Erosion and Landscape Studies tivity measurements as an indicator of forest soil qualityon nonsaline forest soils in Ontario, found that varia-In many regions of Canada, concerns about soil degra-

dation related to erosion have been the impetus for tions in some soil chemical attributes and total N, that


Table 1. Sequential framework to evaluate soil quality for specific purpose or fitness of use (after Carter et al., 1997; Carter, 2000).

Sequencesteps Sequential framework Questions implied by the framework

1 Purpose What will the soil be used for?2 Functions What specific role is being asked of the soil?3 Processes What key soil processes support each function?4 Properties or attributes What are the critical soil properties for each process? What are their critical

or threshold levels?5 Indicators, surrogates, or pedotransfer function When the attribute is difficult to measure or not available, which indirect

or related property or properties can be used in its place?6 Methodology standardization What methods are available to measure the attribute? Technical rules and

protocols for soil sampling, handling, storage, analysis, and interpretationof data.

were major determinants of forest productivity, ac- those factors apart from soil that influence crop yield,such as climatic (i.e., precipitation, evaporative demand,counted for most of the variation in apparent electrical

conductivity of bulk soil. and air temperature), topographic, and hydrologic pa-rameters. Generally, inherent soil quality for crop pro-duction cannot be evaluated independently of extrinsicDESCRIPTIVE AND FUNCTIONALfactors. For example, a high clay content may be favoredAPPROACHES TO SOIL QUALITYin a semiarid region, where soil moisture retention is

The above Canadian studies generally provide a de- an advantage, but may be undesirable in humid condi-scriptive approach to soil quality based on changes in tions in which poor internal drainage may limit yieldssoil properties and components rather than a functional (Janzen et al., 1992). In similar fashion, a certain soilapproach based on the value or fitness of a soil for a bulk density can be optimal under a semiarid moisturespecific use. In response to the need to characterize the regime but deleterious under a humid moisture regimefunctional approach, Anderson and Gregorich (1984) due to changes in relative saturation and subsequentproposed that soil quality be defined as “the sustained poor soil aeration (Carter, 1990). Because of these con-capability of a soil to accept, store, and recycle water, siderations, there is no universally applicable set of in-nutrients, and energy.” Other studies also emphasized herent soil quality criteria and optimum values.the need to assess soil quality on the basis of soil func- Inherent soil quality can be assessed using nationaltions (Larson and Pierce, 1991, 1994; Doran and Parkin, land resource or soil survey inventories. MacDonald et1994; Doran et al., 1996; Karlen et al., 1994, 1997). On al. (1995) developed an inherent soil quality index forthe recognition that agriculture is part of a much broader some Canadian soils based on soil drainage status, wa-ecological system, Acton and Gregorich (1995) stressed ter-holding capacity, cation exchange capacity, physicalenvironmental concerns by defining soil quality as “the rooting conditions (soil depth, crusting, and compactsoil’s capacity or fitness to support crop growth without layers), and chemical rooting conditions (pH and salin-resulting in soil degradation or otherwise harming the ity). Land resource databases were further utilized toenvironment.” Carter et al. (1997) summarized the evo- characterize areas where inherent soil quality was atlution and development of the approaches used in defin- risk based on soil and landscape indicators and indica-ing soil quality. Present-day approaches generally use tors that reflect intensive agricultural practices, both ofan ecological framework to evaluate soil quality based which feature SOM.on the following sequence: functions, processes, attri-butes or properties, attribute indicators, and methodol- Dynamic Soil Qualityogy (Table 1).

Dynamic soil quality encompasses those soil proper-Soil quality has two parts: an intrinsic part coveringties that can change over relatively short time periodsa soil’s inherent capacity for crop growth and a dynamic(e.g., SOM, labile SOM fractions, soil structural compo-part influenced by the soil user or manager. The distinc-nents, and macroporosity) in response to human usetion between inherent and dynamic soil quality can alsoand management and that are strongly influenced bybe characterized by the genetic (or static) pedologicalagronomic practices. Soil organic matter is both inher-processes vs. the kinetic (or dynamic) processes in soilent, as total SOM is related to particle size distribution,as proposed by Richter (1987).and dynamic, as it is related to ongoing inputs of organicmaterial to the soil.Inherent Soil Quality

The quality of any soil depends in part on the soil’s SOIL QUALITY AND SOILnatural or inherent composition, which is a function of ORGANIC MATTERgeological materials and soil-state factors or variables(e.g., parent material and topography). Attributes of Soil organic matter is considered to be a key attribute

of soil quality (Larson and Pierce, 1991; Gregorich etinherent soil quality, such as mineralogy and particlesize distribution, are mainly viewed as almost static and al., 1994) and also environmental quality (Smith et al.,

2000). It is involved in and related to many soil chemical,usually show little change over time.Characterization of inherent soil quality for crop pro- physical, and biological properties. Thus, information is

needed on the multifunctional role of SOM in soil qual-duction also involves consideration of extrinsic factors,


Table 2. Selected attributes of soil organic matter (SOM) that are sensitive to soil management and indicate change in total SOM.

Percent ofOrganic matter attribute soil organic C† Methodology Comments

Macroorganic matter C and N 15–45 Soil dispersion or sieving Sand-sized organic material consisting mainly of fine roots and large(particulate C and N) organic debris.

Light fraction C and N 3–45 Soil densimetric fractionation Composed mainly of plant and microbial residues. A transitory poolof organic material between fresh residues and humidified SOM.

Microbial biomass C and N 1–3 Soil fumigation or extraction The dynamic, living microbial component of the soil, which regulatesthe transformation of SOM and functions as both a sink and sourceof plant nutrients.

Mineralizable C and N 3–30 Soil incubation Reflects metabolic activity of heterotrophic microbes and provides anintegrated, direct measure of SOM turnover.

† Estimates compiled from data in Carter and Rennie, 1982; Carter et al., 1998.

ity, including the setting of critical values and standard- in Canada. Larson and Pierce (1994) and Pierce andGilliland (1997) identify both computer models (whichization.use attributes as variables) and statistical (i.e., temporalpattern of attribute mean and standard deviation) con-Soil Quality and Soil Organic Matter

Attributes and Indicators trol as a means to assess soil quality change over time.Other approaches are use of archived soil and plantSoil quality attributes can be defined as measurablesamples from long-term experiments and use of geostat-soil properties that influence the capacity of the soil toistical methods (Wendroth et al., 1997).perform a specific function (Acton and Padbury, 1993).

One major area of recent research has been the evalu-In many cases, the specific property may be difficult toation of the sensitivity of SOM fractions or attributesmeasure directly, so an indicator is used to serve as anto changes in total SOM. Early studies showed thatindirect, practical measure of the attribute. For dynamicSOM attributes (e.g., microbial biomass and mineraliz-soil quality, indicators are most useful when they indi-able C) were very sensitive to changes in total SOMcate or measure change in the attribute.and could be utilized, based on their relative simple andVarious studies have attempted to identify sets ofstraightforward methodology, as indicators of changeattributes or properties that can characterize a soil pro-(Carter and Rennie, 1982). More recently, a greatercess or processes in regard to a specific soil function.range of labile SOM attributes (e.g., light fraction andArshad and Coen (1992) identified several soil physicalmacroorganic matter), along with a relatively simpleand chemical properties that could serve as attributes ofmeasure of soil structural stability (e.g., aggregate stabil-soil quality. More recently, Topp et al. (1997) providedity), have been evaluated on their sensitivity to changea detailed assessment of soil physical attributes basedin total SOM (Campbell et al., 1989, 1997, 1998a, 1998b;on the ability of the soil to store and transmit liquids,Biederbeck et al., 1998; Bolinder et al., 1999; Angerssolutes, gases, and heat; water parameters; aeration;et al., 1999) (Table 3). Macroorganic matter and lightstrength; and structure. Soil biological attributes werefraction (Gregorich and Janzen, 1996) and soil microbialcategorized by Visser and Parkinson (1992) and Gre-biomass (Carter et al., 1999) are highly responsive togorich et al. (1994, 1997).changes in C inputs to the soil and can provide a measur-Measurement of SOM is relatively straightforward,able change before any such change in the total SOM.so there is little need for an indicator to assess its statusHowever, the validity of this approach to indicate thein soil at any one time. However, it is difficult to measuredirection of SOM is restricted under conditions wheresmall changes in soil organic matter against a relativelyclimate impedes adequate C inputs or suppresses thelarge background mass, so indicators (e.g., major attri-rate of decomposition (Janzen et al., 1998).butes of SOM) are needed that are more sensitive to

change in organic matter inputs than the total mass that Table 3. Comparison of sensitivity analysis of several attributeswill indicate the direction of change in total mass of or fractions of soil organic matter (SOM) to changes in total

SOM in eastern and western Canada.†organic matter (Table 2).Identifying key soil attributes that are sensitive to soil MO-C MO-N LF-C LF-N MIN-C MIN-N MB-C

functions allows the establishment of minimum data setsGrain and forage sequences, 3 to 67 yr (Bolinder et al., 1999)that will provide a practical assessment of one or several

1.3 1.5 1.5 1.6 – – 1.5soil processes of importance for a specific soil functionPotato cropping sequences, 9 yr (Angers et al., 1999)(Larson and Pierce, 1991, 1994). For the multifaceted

– – 1.6 1.7 – – 1.8role or function of SOM, different minimum data setsGrain sequences, 11 yr (Campbell et al., 1997)can be developed to provide a compilation of subattri-

– – – – 1.6 1.4 1.3butes (Gregorich et al., 1994).Grain–green manure sequences, 6 yr (Biederbeck et al., 1998)

– – 1.3 – 1.4 1.6 –Sensitivity of Soil Organic Matter Attributes† MO, macroorganic matter; LF, light fraction; MIN, mineralizable; and

In many cases, soil quality assessment requires a mon- MB, microbial biomass. Sensitivity analysis is based on ratio of a specificC fraction under conservation system (e.g., crop residues retained, useitoring system to provide regular surveillance of soilof forages in rotation) to that under conventional cropping system (i.e.,quality attributes or indicators over time. Wang et al. low return of generation of organic matter). The sensitivity ratio fortotal SOM was 1.1 to 1.2 in all of the above studies.(1997) describes such a nation-wide system established


Fig. 1. Relation between soil organic C and soil erodibility (i.e., tur- Fig. 2. Relation between soil organic C and macroaggregation overtime in a Chernozemic (Boroll) soil. Maximum macroaggregationbidity) in a Vertisol (Cryert) and Ferrallitic (Oxisol) soil. There is

a critical limit of �20 g C kg�1 to reduce erodibility in the Vertisols. is �40 g C kg�1. Data derived from Jastrow and Miller (1997).Data derived from Feller et al. (1996).

1996) to establish an initial reference level ofthreshold (Arshad and Martin, 2001) based on gen-Other approaches have taken a wider approach usingeral consensus.organic matter molecular components, molecular biol-

2. Some studies have set or characterized critical lev-ogy, and soil microbial diversity to complement mea-els for total SOM based on empirically derivedsurement of SOM fractions (Monreal et al., 1997a). How-relations between SOM and specific soil processesever, the utility of such measurements in evaluating soiland conditions. Close relations have been quanti-organic matter quality is not so clear. Kay et al. (1997)fied between SOM and soil fertility indices (Fellerassessed the sensitivity of soil structural characteristicset al., 1996), crop productivity (Bauer and Black,to changes in SOM.1994; Reeves, 1997), soil erodibility (Feller et al.,1996; Fig. 1), and soil aggregation (Angers andLimitations in Assessing SoilCarter, 1996; Jastrow and Miller, 1997; Kay, 1998;Organic Matter QualityFig. 2).

The ecological framework used to evaluate soil qual-The above relationships suggest that critical levelsity (Table 1) can also be utilized to assess SOM quality.

could be established for specific soil types and soil man-The main limitations to this approach are the setting ofagement scenarios. Generally, critical levels for totalcritical limits, or threshold values, and standardizationSOM would be soil or site specific, related to a singlein methodology.soil process or function, and based on a range ratherthan a set value.Critical Limits or Threshold Values for Soil

Organic Matter Standardization for Soil Organic MatterCritical (threshold, trigger, baseline, or reference) limits Standardization concerns the technical protocol for

in soil quality assessment refer to the specific value or the sampling, storage, laboratory methods, and interpre-range of a soil property or indicator that is required to tation of soil properties, attributes, and indicators as aensure that a soil process or function is not restricted means to provide reliable and comparable methodologyor adversely influenced. Based on the primary concept for soil quality assessment (Nortcliff, 1997). At present,of fitness for use for soil quality, critical limits denote there is a need to develop acceptable standard samplingthe boundary values, or margins of tolerance, required and measurement protocols to monitor and evaluatefor soil indicators that are associated with optimum soil change in SOM. Estimates of SOM status and changefunctioning and indicate if a soil is fit for a specific are still mainly based on C concentration rather thanuse (Larson and Pierce, 1994). Few attempts have been mass, and comparisons between treatments are derivedmade to characterize critical values for key soil quality from unequal soil depth, densities, or soil mass (Ellertindicators, such as SOM (Arshad and Martin, 2001), and Bettany, 1995).while limiting values, or thresholds, for fractions and Generally, there is no well-accepted operational defi-components of SOM are generally not known (Carter et nition of SOM (Agric. Soils Working Group, unpub-al., 1999). Soil organic matter serves many soil functions lished, 1999). It is not clear if a measure of SOM should(Gregorich et al., 1994), so the critical value or range include plant litter, crop residues, or root material. Esti-would vary according to function. Generally, at present, mated annual straw and root C inputs of cereal cropstwo main approaches are utilized: in eastern Canada (Bolinder et al., 1997) ranged from

2 to 5% of the total (0–60 cm depth) SOM (Carter et1. Most studies advocate use of average or baselineSOM values under local soil conditions and soil al., 1998) and is probably higher for grasses and legumes.

Crop-derived C in SOM estimates has implications fortypes (e.g., MacDonald et al., 1995; Gomez et al.,


Table 4. Functions of soil organic matter (SOM) in both whole and aggregated soil and relations to soil processes and conditions.

Location of SOM Function or processes involved Some resulting soil conditions

Related to SOM in the whole soil Compactibility and strength (Soane, 1990) Increased resistance to soil compaction pressures (Soane, 1990)Water-holding capacity (Hamblin and Potential increase in plant available water

Davies, 1977) (Johnston, 1986; Hudson, 1994)Soil friability (Christensen and Johnston, 1997; Improve soil workability and decrease draft requirements

Watts and Dexter, 1998) (Low and Piper, 1973)Related to SOM in particles Conservation of nutrients and energy Reduced turnover time for SOM and increased retentionand aggregates (Christensen, 1996; Hassink, 1997) of nutrients (see Table 5; Kay and Angers, 2000)

Soil physical processes (Kay, 1998) Stabilized pore size distribution; improved infiltration of waterin soil and aeration status (Thomasson, 1978; Kay andAngers, 2000)

Erodibility (Feller and Beare, 1997; Enhances stability of soil aggregation and decrease in lossGregorich et al., 1998) of fine soil particles (Feller et al., 1996)

soil-handling protocols after sampling. Use of sieving and C/N ratio) can also influence the quantity of SOM(Juma, 1993).methods to separate crop residue and macroorganic

matter would be beneficial for SOM characterization. Addition of organic matter to the soil, in the form ofcrop residues or organic amendments, increases thelevel of low-density macroorganic matter, which canFACTORS THAT REGULATE ORGANICrepresent up to 45% of total SOM (Carter et al., 1998;MATTER FUNCTIONING IN SOILKay, 1998). This form of SOM functions in improving

A major component of sustainable land use is to sus- the mechanical properties of soil (Table 4).tain and improve the quality of the soil resource base.Monitoring is important, but the usefulness of the data Influence of Aggregated Organic Matterwill only be realized if it is used in management decisions on Soil Functionsto correct deficiencies or improve the quality of the soil

The capacity of a soil to store organic matter is relatedresource (Pierce, 1996). In regard to SOM, monitoringto the association of SOM with clay and clay plus siltwould indicate if levels in a specific soil are in decline(2–20 �m diam.) particles, soil microaggregates (20–250or too low to adequately support those functions or�m diam.) and macroaggregates (�250 �m diam.), andprocesses that are SOM dependent. Evidence of lowthe fraction of sand-sized macroorganic matter (Chris-SOM could result in the following soil managementtensen, 1996; Tisdall, 1996). Soil mineralogy and particledecisions: (i) Assess if the soil management system issize distribution regulate the capacity of a soil to pre-capable of producing or providing adequate organicserve organic matter and control soil aggregation. Thematter inputs (i.e., inputs of crop residue, organic amend-interrelation between soil aggregation and SOM con-ments, or both) to prevent a decline in SOM and (ii)strains both decomposition (e.g., separate C substrateassess if present management strategies allow the bestfrom decomposer organisms) and predation (e.g., sepa-use or placement of organic matter inputs.rate microbes from predators) processes, and conse-Factors that regulate SOM functioning in soil arequently results in the conservation and stabilization ofrelated to organic matter additions or inputs, which in-SOM (Juma, 1993).fluence particulate or macroorganic matter, and the re-

Arable soils contain less organic matter than adjacentlation between SOM and soil aggregates. Functions ofSOM, differentiated on the basis of total SOM or aggre-gated SOM, are given in Table 4.

Influence of Organic Matter Inputson Soil Functions

Increasing organic matter inputs via agricultural man-agement is the key to increasing SOM quantity (Janzenet al., 1997). If the initial level of SOM is below thecapacity of a specific soil to store organic matter, thenSOM levels increase linearly with increasing input levelsalthough the slope of the line may differ, reflecting thevarious influences of climate, soil type, and soil manage-ment (Fig. 3). The major management strategies to in-crease SOM quantity are increasing primary produc-tion (e.g., perennial crops, plant nutrition, and organicamendments) and increasing the proportion of primaryproduction returned to or retained by the soil (e.g., cropresidue retention and placement). Placement of residuesat depth can also allow a relative increase in stored Fig. 3. Comparison of change in soil organic C in relation to totalSOM (Carter et al., 1998). In some cases, vegetative organic C inputs at three different locations (after Parton et al.,

1996).differences and quality of crop residue (e.g., lignin, N,


grassland soils, but the amount of SOM associated withthe clay plus silt (i.e., 2–20 �m diam.) can be similar(Hassink, 1997). More than 80% of the organic matterin temperate soils can be associated with �20-�m-diam.organomineral particles (Christensen, 1996). However,the capacity of a soil to protect and accumulate SOMin organomineral particles is not always positively re-lated to the clay and silt content per se but rather tothe degree to which this capacity level is filled (Hassinkand Whitmore, 1997). Once the clay plus silt is saturatedwith organic matter, additional SOM would be foundin macroaggregates (Angers and Carter, 1996; Franz-luebbers and Arshad, 1996), probably as sand-sized mac-roorganic matter. Figure 4 illustrates the above differ-ences in the association of soil organic C in whole soil,organic matter fractions, and aggregates for a Char-

Fig. 4. Association of soil organic C in whole soil, organic matterlottetown fine sandy loam (Haplorthod) under two crop-fractions, and aggregates, and changes in water-stable aggregates inping systems in eastern Canada and indicates the im-a Charlottetown fine sandy loam (Haplorthod) under two croppingportance of these processes on changes in water-stable systems. Organic C in clay plus silt particles estimated using the

aggregates. model of Hassink and Whitmore (1997). Error bars indicate stan-dard error.Grassland and forest soils, which can contain rela-

tively high amounts of organic matter, generally havemore sand-sized organic matter than arable soils (Carteret al., 1998). In well-aggregated soils, most of the SOM distributes organic matter into fractions with an increas-can be found in macroaggregate complexes (Angers and ing susceptibility to decomposition as follows: withinCarter, 1996; Jastrow and Miller, 1997). Figure 5 illus- clay plus silt particles, within microaggregates (i.e., in-trates, using a conceptual model, the SOM content and tramicroaggregate), within macroaggregates but exter-

nal to microaggregates (i.e., includes light fraction, mac-proportion in soil particles and aggregates of a Char-lottetown fine sandy loam, from Prince Edward Island, roorganic matter, and microbial biomass), and free

macroorganic matter (Carter, 1996; Christensen, 1996).and different types of SOM capacity levels related toorganic matter inputs and aggregation. Clay plus silt The aggregation process itself is a means to both con-

serve and protect SOM and allow the stored organicserve as a fixed capacity level (Hassink, 1997; Hassinkand Whitmore, 1997) while the combination of aggre- matter to function as a reservoir of plant nutrients and

energy. This is illustrated by the turnover time for SOMgated C and macroorganic C provide an additional vari-able capacity. The former is soil specific while the latter fractions and SOM in aggregates based on various isoto-

pic studies (Table 5). Aggregated SOM functions intends to be contingent on both soil type and manage-ment (i.e., C inputs). regulating air and water infiltration and soil stability,

and thus can serve as an indicator associated with theThe formation of aggregates in most temperate soils

Fig. 5. Conceptual model of soil organic matter (SOM) content and proportion in soil particles and aggregates of a Charlottetown fine sandyloam (Haplorthod) illustrating different measurable capacity levels. OM, organic matter.


Angers, D.A., and M.R. Carter. 1996. Aggregation and organic matterTable 5. Estimates of turnover time for soil organic matter instorage in cool, humid agricultural soils. p. 193–211. In M.R. Carterdifferent fractions and in soil aggregates (after Carter, 2000).†and B.A. Stewart (ed.) Structure and organic matter storage in

Type of organic matter Estimated turnover time agricultural soils. Lewis Publ., CRC Press, Boca Raton, FL.Angers, D.A., L.M. Edwards, J.B. Sanderson, and N. Bissonnette.Organic matter in fractions years

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Nonaggregated soil 1–7 Arshad, M.A., and S. Martin. 2001. Identifying critical limits for soilMacroaggregates‡ (�250 �m diam.) 1–23 quality indicators in agroecosystems. Agric. Ecosyst. Environ. (inMicroaggregates (20–250 �m diam.) 3–80 press).Silt plus clay (�20 �m diam.) 5–1000 Bauer, A., and A.L. Black. 1994. Quantification of the effect of soil

organic matter content on soil productivity. Soil Sci. Soc. Am. J.† Compiled from data in Carter, 1996; Gregorich and Janzen, 1996; Collinset al., 1997; and Monreal et al., 1997b. 58:185–193.

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Bolinder, M.A., D.A. Angers, and J.P. Dubac. 1997. Estimating shootto root ratios and annual carbon inputs in soils from cereal crops.CONCLUSIONSAgric. Ecosyst. Environ. 63:61–66.

Bolinder, M.A., D.A. Angers, E.G. Gregorich, and M.R. Carter. 1999.A wide range of land management studies in CanadaThe response of soil quality indicators to conservation manage-and elsewhere, conducted over the last two decades,ment. Can. J. Soil Sci. 79:37–45.have established that SOM and aggregate stability are Campbell, C.A., V.O. Biederbeck, M. Schnitzer, F. Selles, and R.P.

important indicators to assess sustainable land use. Eco- Zentner. 1989. Effect of 6 years of zero tillage and N fertilizermanagement on changes in soil quality of an Orthic Brown Cherno-system concepts such as functions, processes, attributes,zem in southwestern Saskatchewan. Soil Tillage Res. 14:39–52.and indicators as well as methodology provide a useful

Campbell, C.A., B.G. McConkey, V.O. Biederbeck, R.P. Zentner, D.framework to assess SOM quality although the impedi-Curtin, and M.R. Peru. 1998a. Long-term effects of tillage and

ment of setting critical limits for SOM and standardiza- fallow-frequency on soil quality attributes in a clay soil in semiaridtion in methodology still remain. At present, critical southwestern Saskatchewan. Soil Tillage Res. 46:135–144.

Campbell, C.A., B.G. McConkey, V.O. Biederbeck, R.P. Zentner, S.limits are mainly established by consensus based onTessier, and D.L. Hahn. 1997. Tillage and fallow frequency effectsreference values derived from soil resource inventorieson selected soil quality attributes in a coarse-textured Brown Cher-although the development of empirical relations be- nozem. Can. J. Soil Sci. 77:497–505.

tween SOM and specific soil processes and functions Campbell, C.A., A.G. Thomas, V.O. Biederbeck, B.G. McConkey,offer the promise of more precise estimates in future F. Selles, D. Spurr, and R.P. Zentner. 1998b. Converting from no-

tillage to pre-seeding tillage: Influence on weeds, spring wheatstudies. Sensitivity analyses have shown that severalgrain yields and N, and soil quality. Soil Tillage Res. 46:175–185.SOM fractions have the potential to indicate change in

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